Henning Schuhmann

Photoluminescence of carbon nanodots: dipole emission centers and electron-phonon coupling.

Inorganic carbon nanomaterials, also called carbon nanodots, exhibit a strong photoluminescence with unusual properties and, thus, have been the focus of intense research. Nonetheless, the origin of their photoluminescence is still unclear and the subject of scientific debates. Here, we present a single particle comprehensive study of carbon nanodot photoluminescence, which combines emission and lifetime spectroscopy, defocused emission dipole imaging, azimuthally polarized excitation dipole scanning, nanocavity-based quantum yield measurements, high resolution transmission electron microscopy, and atomic force microscopy. We find that photoluminescent carbon nanodots behave as electric dipoles, both in absorption and emission, and that their emission originates from the recombination of photogenerated charges on defect centers involving a strong coupling between the electronic transition and collective vibrations of the lattice structure.

Electron microscopy analysis of crystalline silicon islands formed on screen-printed aluminum-doped p-type silicon surfaces

The origin of a not yet understood concentration peak, which is generally measured at the surface of aluminum-doped p+ regions produced in a conventional screen-printing process is investigated. Our findings provide clear experimental evidence that the concentration peak is due to the microscopic structures formed at the silicon surface during the firing process. To characterize the microscopic nature of the islands (lateral dimensions of 1–3 μm) and line networks of self-assembled nanostructures (lateral dimension of ≤50 nm), transmission electron microscopy, scanning electron microscopy, scanning transmission electron microscopy, and energy dispersive x-ray analysis are combined. Aluminum inclusions are detected 50 nm below the surface of the islands and crystalline aluminum precipitates of ≤7 nm in diameter are found within the bulk of the islands. In addition, aluminum inclusions (lateral dimension of ∼30 nm) are found within the bulk of the self-assembled line networks.

Electric breakdown in ultrathin MgO tunnel barrier junctions for spin-transfer torque switching

Magnetic tunnel junctions for spin-transfer torque (STT) switching are prepared to investigate the dielectric breakdown. Intact and broken tunnel junctions are characterized by transport measurements prior to transmission electron microscopy analysis. The comparison to our previous model for thicker MgO tunnel barriers reveals a different breakdown mechanism arising from the high current densities in a STT device: instead of local pinhole formation at a constant rate, massive electromigration and heating leads to displacement of the junction material and voids are appearing. This is determined by element resolved energy dispersive x-ray spectroscopy and three dimensional tomographic reconstruction.

PARAMETER SPACE FOR THERMAL SPIN-TRANSFER TORQUE

Thermal spin-transfer torque describes the manipulation of the magnetization by the application of a heat flow. The effect has been calculated theoretically by Jia et al. in 2011. It is found to require large temperature gradients in the order of Kelvins across an ultra thin MgO barrier. In this paper, we present results on the fabrication and the characterization of magnetic tunnel junctions with three monolayer thin MgO barriers. The quality of the interfaces at different growth conditions is studied quantitatively via high-resolution transmission electron microscopy imaging. We demonstrate tunneling magnetoresistance ratios of up to 55% to 64% for 3 to 4 monolayer barrier thickness. Magnetic tunnel junctions with perpendicular magnetization anisotropy show spin-transfer torque switching with a critical current of 0.2 MA/cm2. The thermally generated torque is calculated ab initio using the Korringa–Kohn–Rostoker and nonequilibrium Greens function method. Temperature gradients generated from femtosecond laser pulses were simulated using COMSOL, revealing gradients of 20 K enabling thermal spin-transfer-torque switching.

Spin-Transfer Torque Switching at Ultra Low Current Densities

The influence of the tantalum buffer layer on the magnetic anisotropy of perpendicular Co-Fe-B/MgO based magnetic tunnel junctions is studied using magneto-optical Kerr-spectroscopy. Samples without a tantalum buffer are found to exhibit no perpendicular magnetization. The transport of boron into the tantalum buffer is considered to play an important role on the switching currents of those devices. With the optimized layer stack of a perpendicular tunnel junction, a minimal critical switching current density of only 9.3 kA/cm(2) is observed and the thermally activated switching probability distribution is discussed.

Tunnel magnetoresistance in alumina, magnesia and composite tunnel barrier magnetic tunnel junctions

Abstract Using magnetron sputtering, we have prepared Co–Fe–B/tunnel barrier/Co–Fe–B magnetic tunnel junctions with tunnel barriers consisting of alumina, magnesia, and magnesia–alumina bilayer systems. The highest tunnel magnetoresistance ratios we found were 73% for alumina and 323% for magnesia-based tunnel junctions. Additionally, tunnel junctions with a unified layer stack were prepared for the three different barriers. In these systems, the tunnel magnetoresistance ratios at optimum annealing temperatures were found to be 65% for alumina, 173% for magnesia, and 78% for the composite tunnel barriers. The similar tunnel magnetoresistance ratios of the tunnel junctions containing alumina provide evidence that coherent tunneling is suppressed by the alumina layer in the composite tunnel barrier.

Transmission Electron Microscopy Investigations of Metal-Impurity-Related Defects in Crystalline Silicon

This contribution summarizes recent efforts to apply transmission electron microscopy (TEM) techniques to recombination-active extended defects present in a low density. In order to locate individual defects, electron beam induced current (EBIC) is applied in situ in a focused ion beam (FIB) machine combined with a scanning electron microscope. Using this approach defect densities down to about 10cm-2 are accessible while a target accuracy of better than 50nm is achieved. First applications described here include metal impurity related defects in multicrystalline silicon, recombination and charge collection at NiSi2 platelets, internal gettering of copper by NiSi2 precipitates and site-determination of copper atoms in NiSi2.

Electron microscopy analysis of silicon islands and line structures formed on screen-printed Al-doped p + -surfaces

Islands and line networks of aluminum-doped p+ regions formed in a conventional screen-printing process are investigated by a combination of different techniques. To characterize the microscopic nature of the islands (lateral dimensions 1–3 μm) and line networks of self-assembled nanostructures (lateral dimension ≤ 50 nm) advanced transmission electron microscopy (TEM), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray analysis (EDX) are combined. Aluminum inclusions are detected 50 nm below the surface of the islands and crystallographic aluminum precipitates of ≪ 7 nm in diameter are found within the bulk of the islands. In addition, aluminum inclusions (lateral dimension ∼30 nm) are found within the bulk of the self-assembled line networks. Our findings provide clear experimental evidence that the concentration peak generally measured at the surface of screen-printed Al-p+ regions is due to the microscopic structures formed on the silicon surface during the firing process.

Transmission Electron Microscopy Investigation of Self-Organized InN Nano-columns

Semiconductor InN nano-columns have electronic properties which make them a promising candidate for novel photovoltaic devices.